1. Transfer your video to the tablet (see also "How do I transfer my files to the Tablet (Android)?").
2. Open the „Galerie“ app on the tablet.
Note: This app is not installed on older devices. Use the "Google Fotos" app.
3. Open your video.
4. The video will play in infinite loop by default. If the video is not playing in infinite loop, tap the tablet until you see the "three dots" at the top of the screen. Tap the "three dots", open „Einstellungen“ and activate "Videos in Endlosschleife abspielen".